It could be lots of things. Presuming you mean from BW2 to PokeBank, odds are you messed something up. Try reassigning a PID, make sure every last detail of the caught/met area is correct (some Pokémon with certain moves or abilities can only be in certain Pokeballs, remember Dream World Pokémon can only be caught in Dream Balls. If the Pokémon is shiny, click 'create shiny ID' on the ID/SID/OT tab. Also check how all your moves are learned, and if they are all possible to obtain with one another and with your ability, especially in the case of Dream World Pokémon (for example, Gliscor can't have Poison Heal with Defog because Defog is a 4th gen only move, and Poison Heal Gliscor can only be obtained in 5th gen and beyond).
If all fails, you can always send me the .pkm file so I can have a look at it.